1. Field of the Invention
This invention relates generally to electro-optic modulators and, more particularly, to modulators with improved harmonic performance resulting from the use of multiple legs.
2. Description of the Related Art
As the result of continuous advances in technology, particularly in the areas of networking and telecommunications, there is an increasing demand for capacity for the transmission of data. Optical fiber is a transmission medium which is well-suited to meet this demand since optical fiber has an inherent bandwidth which is much greater than metal-based conductors, such as twisted pair or coaxial cable. Existing optical fiber communication systems are typically based on a modulator, a fiber link, and a photodetector. The modulator modulates an optical carrier with the data to be transmitted. The modulated optical carrier is transmitted across the fiber link. The photodetector then detects the modulated optical carrier at the other end of the fiber link, recovering the transmitted data.
Protocols such as OC-3, OC-12, etc. have been developed for the transmission of data over such optical fiber systems. These protocols, however, typically use simple modulation schemes which result in low bandwidth efficiencies. As an example, the OC protocol is based on on-off keying, which is a bandwidth inefficient modulation scheme. In theory, the capacity of optical fiber systems could be increased by the use of more bandwidth-efficient modulation schemes, such as quadrature amplitude modulation. However, these schemes result in more stringent requirements on the performance characteristics of the overall system, including on the modulator. For example, the acceptable harmonic levels for a modulator are lower for quadrature amplitude modulation than they are for on-off keying.
The development of new networking architectures and applications also results in new and different performance requirements. For example, the broadcast television format is based on fixed bandwidth, analog transmission channels separated by guard bands. As a result, modulators used in the distribution of cable television typically have stringent requirements on harmonics which fall within the transmission channel (e.g., requirements on in-band spur to signal power) but may have laxer requirements on the harmonics which fall within the guard bands.
As another example, a data transmission system may frequency upshift the data to be transmitted so that the up-shifted signal occupies less than one octave. This effectively eliminates any requirements on second harmonics. As a result, a modulator which exhibited good in-band performance, even at the expense of higher second order harmonics, would be desirable for such a system. Conversely, increasing the transmission rate of a system results in a wider operating bandwidth for the system. As the bandwidth is expanded, however, more harmonics will fall within the operating bandwidth. As a result, it would be desirable to tailor the modulator's harmonic performance such that these in-band harmonics are reduced.
Many of the performance requirements described above could be realized if the harmonic performance of the optical modulator could be manipulated. Current modulator technology, however, is ill-suited for this purpose.
Standard Mach-Zehnder modulators (MZM) are inherently non-linear and it is difficult to manipulate their harmonic performance. For example, one of their characteristics is that the ratio of the third harmonic to the signal is constant, so that the third harmonic cannot be manipulated independent of the signal. Hence, if an application required elimination of the third harmonic, a standard MZM could not achieve this performance requirement without also eliminating the signal.
As a result, MZM-based systems which have specific harmonic requirements must compensate for the inherent drawbacks of MZMs in some other fashion. In one approach, the non-linearity of the MZM operating curve is compensated for by operating the MZM only over a small portion of the operating curve. This reduces some of the unwanted higher harmonics. However, as noted above, other harmonics (such as the third) are reduced in magnitude only because the signal is proportionally reduced in magnitude. Reducing the signal also results in a lower signal to noise ratio, thus reducing the useful range of such a system.
In another approach, electronics predistort the data to be transmitted in order to compensate for the MZM's non-linearity. Alternately, the data may be simultaneously transmitted through two different MZM's, each with a different harmonic characteristic. The corresponding recovered electronic signals, each with a different harmonic composition, is then combined electronically to cancel the unwanted harmonics. Both of these approaches, however, require additional electronics. In addition, for high speed systems, the electronics may also significantly limit the overall speed and bandwidth of the system.
Modulators other than standard MZMs also suffer from significant drawbacks. For example, many non-standard designs attempt to manipulate the overall modulator performance by creating multiple, different RF signals from a single data stream, modulating the optical carriers in the modulator with these RF signals, and then optically combining the modulated optical signals. The RF signals are selected such that the subsequent optical combining results in the desired performance. For example, one RF signal may be a modulated version of the data stream, while a second RF signal may include an upshifted, carrierless version of the first RF signal. This approach, however, requires additional electronics with the drawbacks noted above. The application of multiple RF signals to the same modulator also introduces difficulties. For example, the electrodes for the RF signals must be precisely matched or the mismatch may limit the overall bandwidth and speed of the system.
Other non-standard modulators have been designed for specific, limited purposes, such as the suppression of the carrier frequency. However, these modulators do not have the flexibility to manipulate the overall harmonic structure. For example, the carrier-suppressing modulators simply suppress the carrier; the harmonic sidelobe structure is unaffected.
Thus, there is a need for electro-optic modulators which can be tailored to meet a required harmonic performance and particularly for such modulators which can be operated at high speeds and large bandwidths.